To better understand the mechanisms of intellectual impairment and emotional disturbances in individuals with schizophrenia, bipolar disorder, mental retardation or autism, it is essential to first study how cortical circuits are assembled during normal brain development. Cajal-Retzius (CR) neurons have long been implicated in cortical development and are thought to play a role in several of these disorders. The best documented function of CR neurons is in orchestrating neuronal migration and cortical lamination. This is achieved through their secretion of reelin, though other reelin-producing cells in the cortex could also play a role. Interestingly, CR neurons show spontaneous correlated activity in the postnatal neocortex and are known to make both chemical and electrical synapses with apical dendritic spines of developing pyramidal neurons. Therefore, it is conceivable that CR neurons could play a role in synaptogenesis by pyramidal neuron dendrites and in the emergence of spontaneous oscillations in cortical neurons. We want to test this hypothesis using in vivo 2-photon imaging of the structure and function of immature cortical circuits. Specifically, we will image CR and pyramidal neurons expressing the green fluorescent protein (GFP) in different lines of transgenic mice, as well as cortical neurons loaded with fluorescent calcium indicators to detect neuronal firing. We have identified a line of transgenic mice (Ebf2-GFP) in which GFP is expressed in virtually all CR neurons. In addition, we obtained a line of mice (Ebf2:GFPiCre) that expresses the Cre recombinase in the same cells. Using state-of- the-art 2-photon microscopy, we intend to selectively kill CR neurons, or genetically silence their activity using optogenetics and Cre-Lox technologies, and then examine the repercussions of such targeted perturbations of CR neurons on neocortical development. We will investigate whether CR ablation/silencing affects the maturation of apical dendrites and spines of pyramidal neurons and/or the spontaneous activity of networks of pyramidal neurons. These studies address fundamental questions about cortical development. The experiments are designed to fill knowledge gaps about CR neurons and identify novel roles for these cells in cortical circuit assembly. These data should resolve longstanding controversies about their function that arose from methodological shortcomings of prior studies. Our work will also generate new ideas about how subtle defects in cortical circuits might contribute to several neuropsychiatric disorders and thereby help to find improved treatments for these devastating diseases. PUBLIC HEALTH RELEVANCE: The proposed studies will investigate how brain circuits are assembled during development in areas important for emotion, cognition and creativity, as well as for learning and memory. The experiments are designed to generate new ideas about how subtle alterations in brain wiring could result in devastating neuropsychiatric disorders such as schizophrenia, autism, mental retardation or bipolar disorder.